New home on the market with basement Jacuzzi and easy commute to Saturn. Tiny sand grains gathered from Saturn鈥檚 rings reveal that Enceladus, already famous for its impressive water jets, is hiding a warm ocean deep beneath its surface. The finding suggests this moon is the only known place besides Earth with ongoing hydrothermal activity, and boosts the chances of finding life snuggled below its cold exterior.
NASA鈥檚 Cassini probe had already shown that Enceladus sprays plumes of dust and ice kilometres into the air. Combined with gravity data, this points to a subsurface ocean around 40 kilometres beneath the moon鈥檚 south pole. Such an ocean could be a boon to the search for alien life, but anything living there would need a source of energy, because the sun is too far away to provide much warmth.
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Now painstaking detective work by Hsiang-Wen Hsu of the University of Colorado, Boulder, and his colleagues has made a connection between sand grains picked up by Cassini over a decade ago and the moon鈥檚 sea floor.
Cassini isn鈥檛 able to scoop dust from Saturn鈥檚 rings directly, but instead samples material accelerated away from the planet by its magnetic field. Hsu traced some of these grains to Saturn鈥檚 E ring, which is thought to be created by Enceladus鈥檚 plumes.
Rocks under pressure
Cassini鈥檚 Cosmic Dust Analyzer instrument revealed that the dust consists of silica particles between 4 and 16 nanometres across 鈥� like sand, but much smaller. That means the grains couldn鈥檛 have formed from larger particles colliding within the rings, because then you would expect a greater range of sizes.
鈥淚t鈥檚 difficult to explain what we observed because we only see the particles in this narrow size range,鈥� says Hsu.
The team wondered if chemical reactions between rock and water on Enceladus鈥檚 ocean floor could explain the finding. To investigate, they mixed water and silicon-rich rocks commonly found on asteroids and comets, then heated them under pressure for months to replicate conditions within Enceladus.
Out popped grains much like the ones Cassini had caught. What鈥檚 more, they provided a measure of the temperature inside Enceladus: the right kind of particles only formed above 90鈥壜癈, suggesting the icy moon鈥檚 interior must be at least that warm.
鈥淎ll this independent evidence points in one direction, that means these particles most likely form from hydrothermal interactions within Enceladus,鈥� says Hsu.
Central heating
Hsu thinks the moon鈥檚 entire core is probably porous and filled with warm water that diffuses up to the subsurface ocean. But the source of all this heat is still a mystery. Any heat left over from Enceladus鈥檚 formation should have radiated away long ago. Tidal heating, generated by the squeeze of Saturn鈥檚 gravity, wouldn鈥檛 provide enough energy and chemical reactions or radioactive decay that could provide warmth should also have died out by now.
Enceladus may once have received a tidal heating boost from Saturn鈥檚 other moons that we are now seeing effects of, but the details aren鈥檛 clear, says Hsu.
There may be other reasons for the diminutive size of the grains seen by Cassini, says of Washington University in St Louis, but it鈥檚 a promising find. 鈥淚f there is a source of energy, that鈥檚 one of the prerequisites for life,鈥� he says. 鈥淚f the story that they鈥檝e put together is true, then the implications for what鈥檚 going on inside Enceladus are profound.鈥�
Other icy moons are thought to harbour subsurface oceans, but the hydrothermal regions on Enceladus are particularly exciting because they closely match some found in the Atlantic, says Carolyn Porco, head of the Cassini imaging team and also at the University of Colorado, Boulder. Bacteria thrive at such sites on our planet. 鈥淲hen we can draw a connection between an environment on Earth and an environment on Enceladus, that just ups the ante for the possibility we might find life,鈥� she says.
War of the watery worlds
Cassini is due to fly through one of the moon鈥檚 icy plumes later this year in search of molecular hydrogen, which might be produced in the reaction between the rock and water if it is occurring and could be a source of fuel for bacteria. But there is a limit to how much the probe can do. 鈥淎ll of these instruments were designed for another purpose,鈥� says McKinnon. 鈥淲e need to go back with instruments more specifically tuned to the Enceladus questions.鈥�
Unfortunately, a mission to Enceladus is not on the cards. The closest we will get is NASA鈥檚 recently announced mission to Jupiter鈥檚 icy moon Europa. Although Europa is much larger than its Saturnian cousin, there may be similar processes at work inside both, says Hsu. 鈥淲e could apply what we鈥檝e learned here to explore Europa.鈥�
Not everyone agrees Europa is the best way forward 鈥� although it does show signs of plumes, observations are less certain than those at Enceladus. 鈥淚f the goal is to look for things having to do with astrobiology, Enceladus is a far better bet, but the train had left the station a long time ago to conduct a Europa mission,鈥� says Porco.
McKinnon thinks Europa is worth the visit. Jupiter is much easier to reach than Saturn, and we鈥檝e not had a dedicated probe there since the end of NASA鈥檚 Galileo mission in 2003, he says. 鈥淕iven the greater gravity at Europa, it鈥檚 much harder to see plumes there. You could have Enceladus-like activity going on at Europa right now, and you just couldn鈥檛 tell because we don鈥檛 have a machine there.鈥� In the war of the watery worlds, it looks like Europa might just beat Enceladus as the first place we find life beyond on Earth.
Journal reference: Nature, DOI:
